Biomechanical Evaluation of Two Internal Fixation Implants Used for Canine Cervical Spine Arthrodesis
Two fixation devices used to surgically arthrodese the canine cervical spine were evaluated in an in vitro biomechanical study. Twelve cervical spine cadaver specimens (C3-C6) were harvested from skeletally mature neurologically normal dogs. Dual energy X-ray absorptiometry of each spine was obtained to determine bone mineral density for each vertebra. Digital radiographs of the specimens ruled out skeletal abnormalities and confirmed physeal closure. Biomechanical testing of each spine was done as intact specimens and after fixation of the C4-C5 intervertebral space. Spines were randomly distributed between 2 implant treatment groups. Group 1 (n = 6) underwent traditional pin-PMMA fixation by use of positive-profile threaded pins implanted in the vertebral bodies and group 2 (n = 6) underwent screw-bar-PMMA fixation using a novel implant design. In group 2, the construct consisted of cortical bone screws implanted bilaterally in the transverse processes and a reinforcing bar was wired to the screw heads prior to covering with PMMA. Four-point bending was used to approximate a pure bending moment throughout the length of the spine. Each spine was deformed at a constant rate to the same angular deformation. Load deformation curves were recorded during extension during the third through fifth cycles of bending. Stiffness in dorsal bending of unaltered spines was compared to similar measurements in surgically altered spines.
Treated spine specimens were significantly stiffer than controls (P < 0.0001) in dorsal bending (estimated mean difference (treatment--control) = 0.098 Nm/degrees, CI: (0.074, 0.122)). There was no statistical difference in stiffness between the traditional pin-PMMA fixation and the novel screw-bar-PMMA fixation (P > 0.05).